1. Technical Field
The present invention relates to an RFID reader device and an antenna device.
2. Background Information
Antennas are known from “Rothammels Antennenbuch [Antenna Book]”, A. Krischke, 12th edition, 2001. Various forms of antennas are explained on pages 65 to 71. The antenna has the task of converting the conducting wave originating from the transmitter into the sky wave or vice versa, receiving the sky wave from space and converting it into the conducting wave, which is then fed to the receiver. The antenna is a transmission region, which can be conceived as a spread out line. It acts as an adaptation transformer between line and free space. A progressive wave forms in the case of power adaptation, which is desired in the case of transmitting and receiving.
Dual wire lines are explained on pages 107 to 111, which consist of two wires extending in parallel, having a small distance in relation to the wavelength. Dual wire lines, double lines, or also parallel wire lines are constructed symmetrically in relation to ground. Strip lines and microstrip lines are explained on page 112. If a lossless line is terminated at its end with a load resistance, which corresponds to the line surge impedance, the power running toward the termination resistor is consumed completely therein. This case is the ideal adaptation. The adaptation factor is the inverse of the ripple. The adaptation factor assumes the value 1 in the case of adaptation and the value 0 in the case of open circuit or short circuit. According to pages 118, 119, feed lines which transmit high frequencies tend to act as an antenna themselves. The radiation emitted into their surroundings can cause undesired directionality and losses. Radiating feed lines can also induce interference of broadcast radio and television reception. This side effect is typically more unpleasant than the slight radiation loss. The undesired radiation of feed lines is dependent, on the one hand, on the construction of the line and, on the other hand, on the degree of maladjustment to the line, it increases with increasing ripple. Dual wire lines are symmetrical to ground, both individual conductors have equal cross section and identical ground ratios. Therefore, the current flowing in the two conductors are also of equal amount, but oriented in opposite directions. The magnetic fields behave similarly. They would cancel out if both conductors were spatially coincident, which cannot be practically implemented, however. Because of the spatial distance of the two conductors, which is always present, the canceling is not complete. The loss radiation of a dual wire line grows directly with the square of the conductor distance and the operating frequency. This means that the conductor distance should become less with increasing frequency.
Baluns are explained on pages 145 to 155. If a phase rotation of 180° is present, the common mode waves cancel out and the differential mode waves are amplified. A differentiation is made between tuned and broadband baluns, and between non-transforming and transforming baluns. Balancing and transformation are often carried out simultaneously.
A system for RFID communication and for testing is known in U.S. Pat. No. 7,298,267 B2. In this case, an RF source is implemented to provide RF energy to an RFID transformer. The RF source outputs a continuous RF signal via a transmission line in this case. A coupler for coupling to the RFID transponder and the diode are provided separately therefrom, wherein the diode is coupled to the coupler and an interface. The interface is coupled to the diode and is adapted by means of the coupler to modulate the RF energy of the RF source. By way of the separation of the RF source from the interface, which modulates the RF energy by means of the coupler, a simple arrangement having a plurality of couplers is formed, which enables parallel testing of a large number of RFID transponders at high speed.